The Depth Charge Doesn't necessarily have to be done with whisky as the spirit/liqueur, but there isn't a tag for generic 'shot' so I had to settle for my fave.

Anyway, take your shot glass and fill it with a spirit/liqueur of your choice. Then drop it slowly into a full pint of beer. Hopefully (!?), the spirit/liqueur will be denser than the beer and thus remain in the shot glass.

Now drink away merrily at your beer until you reach the last dregs, at which point the shot will come out of the glass, mix with the beer, and create an altogether nice surprise. Yum!

(Note: You can also do this with cider)